i80 



Embryogenesis: Progressive Differentiation 



ceive of the "organizer" material as of a 

 kind of general manager which determines 

 the destiny of the entire remainder of the 

 embryo. Large portions of the embryo are 

 not subject to its influence and, as has been 

 pointed out already, the demarcation and 

 invagination of the "organizer" itself are 

 largely controlled by factors outside this 

 area. 



The "organizer" has suffered further de- 

 valuation. It is true that when part of this 

 area is grafted into another gastrula it not 

 only reorganizes itself into an incomplete 

 axial system but it supplements its own dif- 

 ferentiations by way of "assimilative in- 

 duction," i.e., adjacent host cells (mainly 

 mesoderm) become harmoniously incorpo- 

 rated into the system. Rather than consider- 

 ing this integrating action as a unique prop- 

 erty of the "organizer," one may interpret 

 it as just another demonstration of the char- 

 acteristics of a morphogenetic field. The 

 graft extends its field properties into the 

 adjacent non-determined host cells, probably 

 because it is merely a fragment of the "or- 

 ganizer" and as such in an "vmsaturated" 

 condition. In the unfragmented normal em- 

 bryo there is no necessity for assimilative 

 induction. There, the organizing effect of 

 the "organizer" is confined to its own ma- 

 terial, namely the chorda-mesoderm man- 

 tle.* 



While the "organizer" becomes segregated 

 into distinct tissue primordia it loses the 

 regulation tendencies of a field and, at the 

 same time, the capacity of assimilating ex- 

 perimentally supplied additional material 

 into its system. Yet these primordia, let 

 us say of an advanced neurula, retain strong 

 and complex inductive powers. When ex- 

 perimentally confronted with gastrula ec- 

 toderm, they may induce a complete tail, or 

 brain and sense organs. However, the struc- 

 tural pattern of these inductions has no re- 

 lationship to that of the inductors; it is 

 autonomously organized. Thus the deriva- 

 tives of the "organizer" behave essentially 

 like dead or adult tissues which can likewise 

 induce highly complex organ systems but 

 do not contribute to the patterning of their 

 inductions. 



Field Characteristics of the Inductions. It has 

 been argued that there are two kinds of in- 

 ductive agents: (1) "evocators" (Needham, 



* Some difficulty of interpretation arises in the 

 case of the induction of tail somites by the posterior 

 part of the archenteron roof. This process might be 

 considered as a rather belated assimilative field 

 effect of the "organizer." 



Waddington and Needham, '34), which are 

 supposedly present in normal inductors as 

 well as in dead tissues and certain chemi- 

 cals, and which evoke merely amorphous 

 cell masses; (2) "modvilators" (Waddington, 

 '40) or "eidogens" (Needham, '42), which 

 are present in normal inductors only and 

 are thought to specify the induction so as 

 to acquire organotypical patterns ("indi- 

 viduation"). This concept has been discussed 

 recently by Holtfreter ('51). We shall con- 

 fine ourselves to brief statements which are 

 largely based upon the data presented in 

 the preceding chapters. 



It has been shown that fresh or dead adult 

 tissues can call forth perfectly individuated 

 inductions outside of a whole host, namely 

 in an isolated piece of ectoderm (Figs. 93-96). 

 This excludes the necessity of an interven- 

 tion of "eidogens" for the emergence of 

 highly complex and normal anatomical pat- 

 terns. Since the atypical inductors possess no 

 structure which they have in common among 

 themselves or with the inductions, it seems 

 reasonable to conclude that they simply ac- 

 tivate the ectoderm to establish a new head 

 or tail field which then organizes itself into 

 typical tissue patterns. If there are such sub- 

 stances as individuating "eidogens," they 

 did not come from the inductors but arose 

 de novo within the induced ectoderm. This 

 would make it futile to attempt a distinc- 

 tion between "inducing" and "evocating" 

 agents. 



There is good reason to assume that in 

 normal development the archenteron roof 

 acts not much differently from a dead in- 

 ductor. The neural plate is induced in the 

 form of new complex field systems which 

 have no point-to-point relationship to the 

 inductors and are capable of organizing 

 themselves into anatomical patterns. This 

 independence applies also to secondary in- 

 ductions (lens, ear vesicle), since their 

 complex structures have evidently no coun- 

 terpart in the structural properties of their 

 inductors. Striking evidence for this notion 

 that independent fields and not specific or- 

 gans or structures are induced has been 

 provided by the xenoplastic experiments. 

 They showed that although the parts of the 

 archenteron roof have regionally specific 

 effects, the elaboration of anatomical and 

 histological patterns is due to inherent prop- 

 erties of the reacting material. In this re- 

 spect, therefore, the inductive action of the 

 "organizer" may be compared with the ac- 

 tion of hormones which can stimulate certain 

 tissues to undergo specific differentiations 



